Check the received message value in producer mocks

[jacksilv]

Hi guys, not sure if this is a valid issue.
When developing a unit test for a program that uses the sync/asyc producer, I can only say that I expect to receive a message and that I want the mock response to be success/error.
For my test I need to check the value inside the message, so I propose to extend the producerExpectation struct in mocks.go and to add new methods to the producer mock, that receive in input a string (regexp) and check the received value against this regexp.
I am ok to propose an implementation if it makes sense.

[eapache]

Seems reasonable to me.

[jacksilv]

Thanks folks, I will start working on the implementation soon.